The present invention relates to a method and apparatus for carrying out the deposition of a viscous material on a substrate through the apertures of a stencil or screen, and may be used in a screen printing machine for the deposition of solder paste on a printed circuit board.
As is known, a substrate such as a printed circuit board on which electronic components are placed requires that the components be soldered to the board. A viscous material, such as a non-conductive or conductive adhesive solder paste or another silicon type viscous material is often deposited on the substrate before the component is placed on the substrate. Screen printing machines that are commercially available from a number of sources are used to automatically deposit the viscous materials through a screen or stencil onto the substrate. The screen or stencil can be either of the traditional type, that is to say a mesh or metallic stencil, or of the type which is the subject-matter of WO 96/01743 of the assignee of the present invention, relating to a stencil allowing the deposition of the materials to be applied.
Examples of the implementation of the present invention provided herein are within the area of deposition by screen printing of solder pastes such as are used in the electronics industry to produce printed circuit boards by soldering of components onto the boards. Nevertheless, the present invention can find application in other technical fields where viscous materials are used.
Generally, the solder pastes used in screen printing in the electronics industry are heterogeneous materials, the components of which have different densities, and are composed of metallic materials and organic or flux materials. The mass of the metallic portion of the solder paste represents approximately 85% to 90% of the total mass, with a density of 8 to 12 according to the metals used. It is understood that the term density means the weight as compared to 1 liter of water. In volume, the metallic portion represents only approximately 50% of the total volume. The organic material, also called flux, has a density of approximately 1.
The solder pastes described above are made up of metallic microspheres joined by the flux or organic material. This viscous flux comprises rheologic agents, adhesive agents and cleaning agents which affect the process of assembling components on printed circuit boards. The process, which is well known, involves:
deposition by screen printing of solder paste contacts onto selected portions of the substrate;
placing of component leads on the paste contacts, the adhesive agent of which holds the components to the board; and
reflowing the solder paste in a furnace or oven, which causes the coalescence of the metallic microspheres, and, when cooled, results in the component being fixed on the board at the proper location.
The function of the alloy included in the solder paste is to provide the supply of metal necessary to ensure electrical interconnection between the leads of the components and the printed circuit by soldering. The organic materials in the paste must disappear at the conclusion of the soldering operation. Nevertheless, there is normally a residue of the organic materials which must be cleaned with water or with solvent, which is both costly and polluting.
Solder pastes reportedly having low residue have been developed. In these pastes, the organic part has substantially the same value in volume terms as in the previous pastes described above. Light solvents with low boiling points can be introduced to provide proper rheology or flow charactertistics. Because of this, these solvents become volatile more rapidly during a preheating step, which generally precedes the reflow step described above during the assembly of components onto printed circuit boards. At the end of the reflow step there thus remains little residue. In order to provide a satisfactory adhesive capacity, the light solvents described above are combined with adhesive resins, which become volatile or sublimate in the reflow step.
The cleaning agents for preparation of a surface which is suitable for obtaining satisfactory inter-metal connections occupy a very small part of the total volume of the paste, in order to reduce the residue. As the overall efficiency of the cleaning must not change, the volume efficiency of the active cleaning constituent has to increase in the same proportions as the total amount of residue diminishes.
These developments in low residue solder pastes result, on the one hand, in a greater dilution of the active cleaning constituents in the paste and, on the other hand, in a greater volatility of the additional solvents used. It is therefore necessary that an extremely homogeneous distribution of the active cleaning constituent be obtained within the volume of each deposit when the solder paste is applied to the substrate. If this is not achieved, deposits of the solder paste will be obtained in which the efficacy of cleaning, for example, will not be identical for adjacent areas. Certain areas will have too much cleaning constituent applied, resulting in cleaning and residue problems. Other areas will not have sufficient cleaning constituents, and therefore the soldering obtained will be of poor quality.
The high degree of solvency and solubility of the additional solvents used results in the evaporation thereof while printed circuit boards are produced. In prior art modes of deposition, the material is dragged by means of an inclined wiper (see the prior art system shown in FIG. 1) in the open air and therefore the evaporation problem is not solved. The evaporation results in a change in the rheology of the solder paste during production, which leads to the parameters for control of the machine being modified. In extreme cases, the solder paste may become too dry and no longer pass properly through the apertures in the stencil.
These problems are exacerbated when production requirements necessitate high-speed screen printing, for example at 200 millimeters per second as opposed to 20 to 50 millimeters per second. To counteract these problems, thixotropic additives are introduced and combined with the other solvents. Evaporation of the base solvents therefore modifies the possible speed of deposition. By way of example, an evaporation of 1% from the volume of solder paste completely changes the rheology and makes screen printing very difficult, if not impossible.
Another of the problems caused by the known technology is the control of the wear and tear on the wiper system. Progressive erosion of the active edge of the wiper by rubbing alters the intrinsic qualities of the paste applied, and that of the depositions, because of the uncontrolled and random retrieval of a certain quantity of microspheres of the metallic portion of the solder with each wipe. In fact, the wipers are normally only changed when the poor quality of the deposition is a noticeable consequence of their wear.
In the prior art, two types of wipers have been commonly used. The first type of wiper is a rubber or polyurethane type. The hardness of this type of wiper varies generally between 70 to 90 Shore. This wiper has the advantage of good deformation by virtue of its low degree of hardness and its flexibility, and therefore good sealing is produced. It has the inconvenience of deforming during passage over the apertures in the stencil. For apertures where the dimension parallel to the wiper is less than 0.5 mm, this is not a major problem. However, where the apertures have dimensions parallel to the wiper greater than this value, the deposit is hollowed out. Where deposits are larger than 3 mm, they are completely dragged off again.
The second type of wiper is a metallic type. The advantage of this type of wiper is its ability to maintain rigidity and therefore not allow the deposit to be hollowed out. The hardness of this type of wiper, however, despite its flexibility, does not allow for perfect sealing with the stencil. The hardness of the metallic wiper sometimes exceeds that of the stencil and therefore often scratches the stencil resulting in encrustation of solder microspheres. The excessive pressure of the metallic wiper can also cause crushing of the tinlead spheres, this alloy being much softer than the steel wiper.
FIG. 1 shows a prior art implementation for depositing a viscous material onto a substrate 1 through a stencil or a screen 2 provided with apertures or openings 3, by means of a wiper 4. The material to be deposited is labeled 5.
In FIG. 1, standard wiper 4, inclined at an angle which can vary from 60xc2x0 to 45xc2x0 with respect to the substrate 1, fulfils at least two functions at the same time. First, it drags the material to be deposited over the stencil (in FIG. 1, in the direction of the arrow 4A). Second, it transfers the material through the apertures or openings 3 in the stencil or screen 2.
The force of transfer, however, can only be exerted if there is displacement of the wiper 4. Furthermore, this force is not constant over the whole length of the wiper 4, but rather is at its maximum at the ends of the wiper 4 and decreases along the length thereof. Because of this differential in force, the result of the transfer is directly linked to the viscosity of the material (which changes quickly), the force of transfer resulting from the sloping of the wiper 4, and from the movement of the wiper 4.
At the point of contact between the wiper 4 and the stencil 2, the wiper 4 fulfils three functions: (1) sealing between the stencil and the wiper; (2) wiping the stencil 2, which allows removal of the surplus material; and (3) contact between the stencil 2 and the substrate 1, there being no contact downstream and upstream of the wiper 4.
The fact that a single wiper 4 fulfils all of these functions makes independent action with respect to each of these functions impossible within the prior art technology. Moreover, the prior art technology has several disadvantages. Referring to FIG. 1, the material to be distributed through the apertures 3 is always downstream of the wiper 4. As a result, as shown in FIG. 1, when the filling of the aperture 3 takes place, it is always in a zone where the stencil 2 is not in contact with the substrate 1. Therefore, the material can be pushed in between the stencil 2 and the substrate 1 (designated as item 5A in FIG. 1), making on the one hand undesirable lines on the substrate 1, and on the other hand fouling the stencil 2, which must be cleaned frequently.
WO 96/20088 filed the Ford Motor Company relates to a method and an apparatus for distributing a viscous material by compression thereof through the apertures of a stencil. The apparatus comprises a reservoir receiving a charge of viscous material; a pressure is exerted on the viscous material in the reservoir. The reservoir is linked via a conduit to a distribution nozzle or compression head having a conical internal shape with baffle plates. The distribution nozzle is provided with a rectangular distribution slit delimited by two wipers disposed in opposite directions, slightly inclined with respect to the vertical. The two wipers bear against the stencil and keep it in contact with the substrate in the zone between them. The aim of this apparatus is to allow implementation of high-speed screen printing.
It appears that the technology disclosed by WO 96/20088 not only does not permit the resolution of the problems described previously, but moreover accentuates them. Indeed, the viscous material has to be placed in a reservoir which is an integral part of the system. The viscous material must, following the reservoir, be pushed under pressure towards the nozzle. The system according to WO 96/20088 has disadvantages, in particular with regard to the cleaning of the conduit from the reservoir to the nozzle. Further, the conical internal shape of the nozzle and the baffle plates with which it is provided, in theory provided to guide and equalize the pressure, will have the effect of laminating the paste. Such an effect is hardly compatible with the heterogeneous nature of the paste and the difference in density of the metallic parts and of the flux. Furthermore, this laminating creates a significant risk of separating the components of the paste and thus results in deposits of unequal quality. In addition, according to WO 96/20088, the nozzle and wipers bear upon the stencil either under the effect of a pressure independent of the pressure applied to the material in the nozzle or under the effect of springs acting on the wipers.
The present invention provides a method and apparatus for carrying out the deposition of a viscous material on a substrate through the apertures of a stencil or screen which allow the above problems to be solved.
According to an aspect of the invention, an apparatus for depositing a viscous material onto a substrate through openings formed in a stencil positioned over the substrate, includes a receptacle having a top and a bottom, and first and second longitudinal sides opposite one another for holding the material, the bottom having an aperture; a piston movable within the receptacle between the top and the bottom of the receptacle for applying a pressure to move the material toward the aperture; and a first wiper attached to the bottom of the receptacle at the first longitudinal side and a second wiper attached to the bottom of the receptacle at the second longitudinal side, wherein the first and second wipers are inclined at an angle with respect to a plane that is parallel to the bottom of the receptacle and wherein when the material is moved toward the aperture, the material is pushed against the first and second wipers and the first and second wipers are pushed against the stencil.
According to another aspect of the invention, a method for depositing a viscous material onto a substrate through openings formed in a stencil positioned over the substrate includes placing the material into a receptacle having a bottom and first and second longitudinal sides, the bottom having an aperture; providing a first wiper attached to the bottom of the receptacle at the first longitudinal side and a second wiper attached to the bottom of the receptacle at the second longitudinal side, wherein the first and second wipers are inclined at an angle with respect to a plane that is parallel to the bottom of the receptacle; applying a pressure and moving the material toward the aperture, wherein when the material is moved toward the aperture, the material is pushed against the first and second wipers and the first and second wipers are pushed against the stencil; and displacing the receptacle and the first and second wipers along the stencil in a direction substantially perpendicular to the first longitudinal side of said receptacle.
According to another aspect of the invention, a cartridge for holding solder paste for use in an apparatus for depositing solder paste onto a substrate through openings formed in a stencil positioned over the substrate, wherein the apparatus for depositing solder paste has means for applying pressure downwardly on the material in the cartridge, includes a lower portion having a plurality of apertures at the bottom thereof; and an upper portion that allows for the means for applying pressure to move the material toward the plurality of apertures; wherein when pressure is applied downwardly on the material by the means for applying pressure in the apparatus, the material passes through the aperture and through the openings in the stencil.
The present invention allows for the various functions to be separated from one another, that is to say the dragging of the viscous material, the transfer of the viscous material and the wiping of the excess viscous material, while providing enhanced sealing and quality of deposits, and also providing solutions with regard to the problems posed by the variations in rheology of the material or by its heterogeneous nature.
A method for making deposits of a viscous material on a substrate through the apertures in a stencil or screen according to the present invention involves containing a quantity of viscous material in a hollow receptacle. This hollow receptacle and its members and accessories, which will be described later, is designed as a disposable or recyclable container (for factory filling) delivered directly from the factory with a quantity of viscous material therein to be deposited. This arrangement avoids the necessity of handling the bulk viscous material, for example by using a spatula, from the pot in which it is traditionally delivered. As the apparatus is designed to be installed directly onto a screen printing machine for producing circuit boards, it can be removed either when it is empty or after use.
According to a method of the present invention, a pressure is exerted on the viscous material, which causes the material to be pushed towards a lower aperture in the receptacle directed towards the stencil. The length of this aperture is preferably adapted to the dimensions of the substrate on which the different deposits are to be made, or of the zone of the substrate on which the deposits are to be made; the width of the aperture is adapted to the desired speed of displacement of the apparatus on the stencil and thus to the speed at which the screen printing is to be done. Thus, a high printing speed can be obtained without adding additives to the paste to modify the rheology of the paste.
The lower aperture of the receptacle containing the viscous or pasty material is delimited or defined by sealing and wiping members. The pressure exerted on the material in the receptacle contributes to exerting a pressure, by means of the sealing and wiping members, onto the stencil and the substrate. As described hereinafter, this pressure contributes both to making a zone of contact between the stencil and the substrate, and to pressurizing the sealing members and wiping off the excess material. In addition, the stencil and the circuit board are in contact with one another in the area between the wiping members where the material is being deposited, thus ensuring good transfer of the material through the apertures of the stencil without allowing the material to pass under the stencil (see item 5A in the prior art, shown in FIG. 1) and without excess material being left on the stencil. Because the apparatus moves laterally along the substrate, there remains an area of non-contact upstream following passage of the sealing and wiping members.
According to a method to be described, the pressure exerted in the receptacle on the material remains constant as far as the distribution aperture and contact with the stencil. This pressure in the apparatus is exerted by the material upon the sealing and wiping members which delimit or define the delivery aperture and are transverse to the direction of downward delivery of the material. This orientation contributes to increasing the sealing function.
Moreover, according to a method to be described, the surface of the material on which the pressure is exerted is larger than the distribution aperture delimited by the sealing and wiping members. A pressure is exerted by the material upon the stencil at the level of the aperture and a pressure is exerted by the material on the sealing and wiping members.
The uniformity of the amount of pressure exerted in the cavity of the receptacle or container, the internal walls of which are rectilinear and/or parallel, avoids dissociation of the components of the material because of differences in density of the components.
A method embodying the invention furthermore performs a homogenization step. This phase is carried out under the influence of the pressure exerted on the material in combination with a homogenizing member which will be described hereinafter. Preferably, this homogenization is obtained by dividing the material into parts of identical volume in the zone between the receptacle itself and the distribution aperture delimited or defined by the sealing and wiping members. The action of the wipers as the receptacle is displaced also assists this homogenization as discussed in detail below.
Furthermore, the invention includes a method and an apparatus which are directly adaptable, practically without modification, to existing screen printing machines. The invention eliminates the need to handle the material in the open air thereby avoiding undesired changes in the material.
Furthermore, as will be set out in the description of embodiments to follow, the apparatus may be useable as a consumable, thrown away after the viscous material it contains is used up, providing a solution to the problem of recycling by the suppliers of unused or incompletely used pots of pastes.
In one aspect, a method of deposition of a viscous material on a substrate through the apertures of a stencil or screen placed on the substrate is as follows:
the viscous material is placed in a hollow receptacle acting as the container and provided with a lower aperture orientated towards the stencil on which the receptacle rests, the receptacle being mounted on a screen printing machine;
the lower aperture of the receptacle is delimited or defined such that its length is adapted to the dimension of the substrate or of the zone of the substrate on which one or more deposits are to be made;
this delimitation or definition of the lower aperture is done by sealing members comprising at least one longitudinal flexible wiper which is transverse with respect to the direction of displacement of the receptacle, the wiper being transverse to the direction of displacement of the material towards the aperture and also being transverse to the direction of the pressure to which it is subjected, with the level of the delivery of the material at an angle of between 120xc2x0 and 180xc2x0;
a pressure is exerted in the receptacle upon the material and it pushes the material towards the stencil to distribute it upon the flexible wiper by pressing on the flexible wiper to make it bear on the stencil and to make the stencil bear on the substrate. In addition, the receptacle is displaced laterally along the stencil.
The delimitation or definition of the lower aperture may be done by sealing members comprising at least two flexible wipers, each on either side of the opening, transverse with respect to the direction of displacement of the receptacle, each wiper being transverse with respect to the push exerted on the material, the wipers being orientated opposite to one another towards the aperture and having a slope with respect to the horizontal, in the zone of the aperture, of between 120xc2x0 and 180xc2x0.
According to another embodiment of the invention, the pressure exerted in the hollow receptacle is constant from the piston to the distribution aperture.
According to another embodiment of the invention, the material exerts a pressure upon the wipers contributing to the sealing and to the forming of the contact between the stencil and the substrate.
According to another embodiment of the invention, the material is homogenized by mixing before the material reaches the aperture.
In one embodiment of the invention, the apparatus for deposition of a viscous material on a substrate through the apertures of a stencil placed on the substrate constitute a receptacle containing a quantity of material, the cavity: containing the material having parallel, rectilinear walls in which a piston moves. The material is pushed towards an aperture delimited by sealing members, for example, wipers orientated in opposite directions at an angle of between 120xc2x0 and 180xc2x0 with respect to the horizontal on the deposition side; in the case of wipers at 180xc2x0, the extremity thereof can be beveled or inclined towards the stencil.
According to another embodiment of the invention, a perforated grille is located in the receptacle above the aperture.
It appears that the invention makes use of a method and a apparatus particularly suitable for transferring a viscous material.
The efficacy of the system in transferring a viscous material through an aperture can be characterized by the following coefficient:
K=contact time (T)xc3x97(PT) pressure of transfer/(VI) viscosity
T being the duration for which the material to be transferred is in contact with the aperture.
PT being the pressure to which the material to be transferred is subjected.
VI being the viscosity.
K is a non-unitary value which is proportional to the efficacy of the transfer.
The greater the value K, the more easily the material will be transferred.